Solid materials through which ions can migrate have been vigorously studied for use as conducting materials in electrochemical elements such as battery cells. Those ion conductors have so far been found for Li+, Ag+, H+, F− and other ion species. Particularly, proton (H+) conductors are expected for application in various electrochemical elements such as fuel cells, capacitors and electrochromic display elements, and can be used as an electrolyte in those electrochemical elements.
The proton conductors are required to exhibit a high proton conductivity at around room temperature. Known proton conductors include inorganic substances such as uranyl phosphate hydrate and molybdophosphoric acid hydrate, and organic substances such as ion-exchange polymer membranes which comprise a perfluoroalkane-based polymer having side chains containing a perfluorosulfonic group.
As the organic proton conductors, proton conductive membranes having sulfonic acid as ion exchange groups are well known. However, such organic proton conductors can be used only within a limited temperature range owning to a reversible elimination reaction of the sulfonic groups and a crosslinking reaction associated with the sulfonic acid. This has set an upper limit on the service temperatures of these conductors, and the conductors have accordingly been limited in their heat resistance applicable to fuel cells.